The initiation of sporulation was analyzed in various mutants in the presence of excess glucose and ammonia, conditions under which normal sporulation remains suppressed. When purine mutants were fed with different concentrations of the slowly metabolizable purine precursor aminoimidazole carboxymide (AICA) or guanine mutants were fed different amounts of guanosine, optimal sporulation was obtained when purine synthesis, and in particular guanosine monophosphate (GMP) synthesis, was partially inhibited. 32Pi incorporation into nucleotides demonstrated that under all conditions under which sporulation occurred the concentration of GTP (and GDP) decreased by more than 50%. A number of purine nucleoside analogs such as virazole, psicofuranine, mycophenolic acid and others were shown to induce sporulation. The role of membrane synthesis was examined by the use of mutants deficient in branched-chain fatty acid synthesis.